Many automobiles and work machines, particularly, earth working machines, use a continuously variable transmission (CVT) to drive wheels or tracks for propulsion. An engine provides power to the transmission, which controls the speed and torque applied to the wheels or tracks. The transmission can increase output torque by decreasing the output speed. A transmission can also decrease output torque by increasing the output speed.
A manual transmission only provides a discrete number of fixed gear ratios. In contrast, a CVT provides an infinite number of transmission ratios to generate an output at any speed in its operating range. One example of a CVT is a hydrostatic transmission consisting of a variable speed hydraulic pump and a hydraulic motor. An example of such a hydrostatic transmission is disclosed in U.S. Pat. Nos. 6,385,970 and 6,424,902 to Kuras et al. With this type of transmission, the transmission ratio is adjusted by controlling the displacement of the hydraulic pump. Another example of a CVT is an electric motor and inverter as is used in hybrid-electric cars. In a hybrid-electric system, a gasoline engine is mechanically coupled to an electric generator, which provides electric power to an electric motor. An inverter contains the power electronics that control the output speed and torque of the electric motor—thus the transmission ratio is adjusted electronically by the inverter.
One important function of a transmission is to decrease output speed when the engine picks up a heavy load that causes the engine to lug. For example, if an automobile is driving along a road and suddenly starts climbing a very steep hill, the engine may begin lugging due to the increased load and may eventually stall unless the transmission is downshifted to reduce output speed and increase output torque. If the automobile has a manual transmission, the operator will be required to downshift when he or she senses that the engine is lugging.
U.S. Pat. No. 6,385,970 to Kuras et al. discloses an engine underspeed control system that performs the same function of reducing output speed for a CVT. Specifically, the engine underspeed control system senses when the engine begins lugging and adjusts the transmission ratio of a hydraulic CVT to reduce output speed and increase output torque to prevent the engine from stalling.
Another function of an engine underspeed control system is to adjust the transmission ratio so that the engine is running at an optimal speed condition—i.e., within a range of speeds where the engine is operating most efficiently. The engine underspeed control thus both helps to prevent the engine from stalling when increased loads are encountered and also ensures that the engine is running efficiently.
With work machines, the engine can become loaded much quicker than in an automobile. For example, during slot dozing, a heavy load can be picked up suddenly when the blade is dropped to the ground and begins pushing dirt heavily. When such a heavy load is suddenly encountered, it is important for the engine underspeed control to respond quickly to prevent the engine from stalling.
If an operator requests an excessive increase in machine output speed, the CVT control system may lose its ability to respond quickly to a subsequent request to decrease output speed—the subsequent request to decrease output speed could come from the engine underspeed control or from the operator. More generally, whenever the requested motor speed becomes much greater or lesser than the actual motor speed, the CVT control system may lose responsiveness. This can also cause the engine underspeed control to lose its ability to respond quickly when a heavy load is encountered to prevent the engine from stalling. Furthermore, a request for an excessive increase or decrease in motor speed could cause physical damage to the motor and/or transmission. Thus, what is needed is a system and method for controlling a CVT that maintains responsiveness, prevents damage to the motor and transmission, allows the engine underspeed control to respond rapidly, and allows the transmission ratio to be adjusted smoothly (i.e., without jerking) to keep the engine running at an optimal speed condition. The disclosed system may satisfy one or more of these existing needs.